Influence of the coil in Boost Switching Regulators
Russell McMahon email (remove spam text)
>I'm currently designing a Boost regulator to convert 12V to 14V. I plan to
>use the LM2587 from NS. This reg needs a coil but I can find nowhere the
>influence of this coil.
I assume you probably mean the LM2578 rather than ...87???
The coil is absolutely vital to the design.
I suggest you search for switching regulator design articles on the web.
HThere are many but here is a good one that covers what you need to know
Here is a very simple "what" without much "why".
In a boost regulator there are two stages to the regulation phase.
In stage 1 the coil input connects to Vin and the coil output is grounded.
Current fow in the coil increases until some preset limit is rached or until
a certain time has passed.
In stage two the connection to ground is removed. The current MUST continue
to flow without interuption and the coil polarity reverses so the end which
causes the end which was grounded to now be above Vin and this end is
connected to Vout by a diode to provide output power.
The cycle then repeats.
>Can anybody help me to choose this coil (specially the value). I need 5A
>Input : 12V/7Ah (Pb Accumulator), Output 13.8V /50W
>and next 12V/4Ah (NiCd Accumulators), output 13.8V/50W
The national "Simple Switcher" application notes contain suggested inductor
Without going into design detail at all, you will probably want a coil with
an inductance in the range of 50 to 300 uH depending on various design
As a guide this will probably be a powdered iron core in the 10 to 20mm
outer diameter range with tens of turns on it.
The LM2578 datasheet provides inductor design calculations.
For this level of current you will need to use an external switch.
Consider also using a more modern IC eg a UCC3803 or UCC3805 (improved
versions of a very old design) which has some advantages due to implementing
current mode control. The latter is slightly easier for a beginner to design
with. The data sheets and applications notes provide most of what you want
to know for designs using these.
http://www.unitrode.com (now part of TI)
Search for app note U-133, SLUS270A, & others
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